Wounds (i.e., lacerations, opening, or ulcers) can be either acute or chronic. Acute wounds are typically sharp injuries to the skin involving little tissue loss. Most acute wounds are closed and are healed by bringing the wound edges together. Chronic wounds are wounds that fail, or are slow, to heal completely. Examples of chronic wounds include pressure sores (decubitus ulcers), diabetic skin ulcers, venous stasis ulcers, burn injury and defects arising following tumor excision.
The cellular morphology of a wound consists of three distinct zones: a central wound space, a gradient zone of local ischemia, and an area of active collagen synthesis. Despite the need for more rapid healing of wounds (i.e., severe burns, surgical incisions, lacerations and other trauma), to date there has been only limited success in accelerating wound healing with pharmacological agents.
The primary goal in the treatment of wounds is to achieve wound closure. Open cutaneous wounds represent one major category of wounds, which includes acute surgical and traumatic wounds, e.g., chronic ulcers, burn wounds, as well as chronic wounds such as neuropathic ulcers, pressure sores, arterial and venous (stasis) or mixed arterio-venous ulcers, and diabetic ulcers. Typically, these wounds heal according to the following process: i) inflammation, ii) fibroblast proliferation, iii) blood vessel proliferation, iv) connective tissue synthesis, v) epithelialization, and vi) wound contraction. Wound healing is impaired when these components, either individually or as a whole, do not function properly. Factors that can affect wound healing, include malnutrition, infection, pharmacological agents (e.g., cytotoxic drugs and corticosteroids), diabetes, and advanced age. See Hunt et al., in Current Surgical Diagnosis & Treatment (Way; Appleton & Lange), pp. 86-98 (1988).
Many different products and protocols are available to treat chronic wounds. See for example Jones et al., in British Journal of Plastic Surgery 55:185-193, 2002, which is incorporated by reference in its entirety. These include simple bandages (notably compression bandages), foams and films, gels and colloids, and pharmaceutical products, such as growth factors. Typically wound healing with a moist occlusive dressing is used rather than using dry, non-occlusive dressings. See Winter, Nature 193:293-94 (1962). Today, numerous types of dressings are routinely used in wound healing. These include films (e.g., polyurethane films), hydrocolloids (hydrophilic colloidal particles bound to polyurethane foam), hydrogels (cross-linked polymers containing about at least 60% water), foams (hydrophilic or hydrophobic), calcium alginates (nonwoven composites of fibers from calcium alginate), and cellophane (cellulose with a plasticizer). See Kannon et al., Dermatol. Surg. 21:583-590 (1995); Davies, Burns 10:94 (1983). Certain types of wounds (e.g., diabetic ulcers, pressure sores) and the wounds of certain subjects (e.g., recipients of exogenous corticosteroids) do not heal in a timely manner (or at all) with the use of these wound dressings.
Research has shown that the majority of ulcers can be induced to heal by the application of adequate levels of sustained graduated compression. For patients with venous disease, the application of graduated external compression, by forcing fluid from the interstitial spaces back into the vascular and lymphatic compartments, can help to minimize or reverse skin and vascular changes attributed to blockage or damage to the venous system. There are three types of bandages that are commonly used:
Type I: Lightweight Conforming-stretch Bandages:
These bandages include products which have a simple dressing retention function, and they should conform well to a limb or joint, without restricting movement.
Type II: Light Support Bandages:
These bandages include products used to prevent the formation of edema and to give support in the management of mild sprains and strains.
Type III: Compression Bandages:
These bandages include products which rely on the application of pressure. They are most commonly employed to control edema and reduce swelling in the treatment of venous disorders of the lower limbs. Compression bandages have been divided into four groups according to their ability to produce predetermined levels of compression.
Type IIIa: Light compression bandages are able to provide and maintain low levels of pressure, up to 20 mmHg on an ankle of average dimensions. The clinical indications for products of this type include the management of superficial or early varices, and varicosis formed during pregnancy. In general, they are not suitable for controlling or reducing existing edema, or for applying even low levels of pressure to very large limbs.
Type IIIb: Moderate compression bandages are used to apply compression on the order of 30 mmHg on an ankle of average dimensions. They are indicated for the treatment of varicosis during pregnancy, varices of medium severity, the prevention and treatment of ulcers and the control of mild edema.
Type IIIc: High compression bandages may be used to apply high levels of compression on the order of 40 mmHg on an ankle of average dimensions. Indications for these bandages include the treatment of gross varices, post-thrombotic venous insufficiency, and the management of leg ulcers and gross edema in limbs of average circumference. Products in this category are not necessarily able to achieve these levels of pressure on very large limbs that have been further enlarged by the presence of edema.
Type IIId: Extra-high performance compression bandages are capable of applying pressures in excess of 50 mmHg. The power in these bandages is such that they can be expected to apply and sustain these pressures on even the largest and most edematous limbs for extended periods of time.
Additionally, several pharmaceutical modalities (e.g., administration of zinc sulfate, vitamins A, C, and D, calcium, magnesium, copper and iron), have also been utilized in an attempt to improve wound healing. However, except in very limited circumstances, the promotion of wound healing with these agents has met with little success.
In the mid-1980s, Dr. Howard Green conceived a method for growing human skin cells such as keratinocytes. See Green et al. (1979) Proc. Natl. Aced. Sci. 76:5665. Epicel™, a product based on these methods, is used to treat deep wounds that require grafting (skin replacement), such as those that occur with severe burns. However, because Epicel™ only replaces the lost epidermal layer, it works best in combination with something that restores the dermal layer of the skin. In fact, Epicel™ is not an artificial skin, but rather is a method in which a new epidermis layer is “grown to order” in a laboratory from surgically harvested skin cells taken from an unburned area of the patient. Thus, Epicel™ functions as an autologous graft. See U.S. Pat. Nos. 4,016,036 and 4,304,866.
In very severely burned patients who have little or no remaining intact skin, artificial skin is an extremely useful material that not only covers and protects the wounded area, but that also promotes re-growth of a natural skin rather than of scar tissue.
Many new artificial skins initially used skin from related donors (such as family members having similar genetic markers). However, doing so required the coadministration of powerful immunosuppressant drugs to dampen the patient's immune system so that the graft would not be rejected. Crippling the patient's immune system in this way can pose additional, serious problems for the patient. Instead, the patient's own unburned skin (often from the scalp, which is rarely burned) is commonly used as a source of graft material.
However, using such skin grafts (or even skin taken from cadavers) does not permanently solve the problems. There is also a need for some type of artificial means to recover skin. Using a synthetic product would also offer an advantage in that such a material is free of viruses, bacteria and other pathogens, which can transmit disease.
For example, Ethicon Inc., a Johnson & Johnson company, obtained exclusive marketing and distribution rights to Integra®, a product which contains no living components, and is not itself actually designed to be a replacement skin. Rather, it provides a protective covering as well as a pliable scaffold onto which the patient's own skin cells can “regenerate” the lower, dermal layer of skin destroyed by a burn. See U.S. Pat. No. 5,489,304. Just as living skin is structured, Integra® consists of two layers. The bottom layer, which is designed to “regenerate” the lower, dermal layer of real skin, is composed of a matrix of interwoven bovine collagen and a glycosaminoglycan that mimics the fibrous pattern of dermis. This matrix is then affixed to a temporary upper layer, a medical-grade, flexible silicon sheet that mimics the epidermal, or surface, layer of skin. Integra® is draped over the wound area and is kept there for 2 to 4 weeks, during which time the patient's own cells make their way into the matrix and create a new dermis. The top layer of Integra® is then removed, and a very thin sheet of the patient's own epithelial cells are then applied. Over time, an epidermal layer is reconstructed from these cells.
AlloDerm™, another product on the market, is sold and manufactured by LifeCell Corporation of The Woodlands, Texas. It is produced by removing from cadaver skin all cell components that cause a burn patient's immune system to reject a graft from any other person. A key feature of this process is the preservation, to the greatest extent possible, of the “natural,” three-dimensional structure of the dermis. Properly approximating this scaffold, whether from real dermis (as in AlloDerm™) or artificial dermis (as in Integra®), is crucial to the ability of the patient's remaining cells to regenerate themselves into a new, functioning skin.
Dermagraft (Advanced Tissue Sciences) is a product which is grown under laboratory conditions from human stromal cells (e.g., fibroblasts from neonatal tissue) seeded onto a biocompatible, chemical base known as a scaffold. See U.S. Pat. No. 5,460,939. Typically, such scaffolds are made of polyglycolic acids, which are the basis of many “resorbable” medical materials, such as surgical sutures and surgical glues. When applied to the body, the scaffold breaks down into glycolic acid and lactic acid, which are carried away by the bloodstream and metabolized to carbon dioxide, oxygen, and water.
Another tissue engineered skin, Apligraf®, manufactured by Organogenesis, is a living two-layer skin substitute that mimics the epidermal and dermal layers of skin. Apligraf® is made with two types of living human skin cells—epidermal keratinocytes and dermal fibroblasts. Moreover, Apligraf®, delivers additional cytokines and growth factors not provided by a dermal layer alone. See U.S. Pat. No. 4,837,379.
The current techniques and applications described above all lack certain important characteristics. It is therefore an object of the present invention to provide a three-dimensional cutaneous tissue allograft construct that will overcome one or more of the abovementioned problems.
What is needed is a safe and effective, means for enhancing the healing of wounds that can be used without regard to the type of wound or the nature of the patient population.